For prometheus use the plain value, not a pointer and change all usages.
Allow AXFR to be requested over udp as well and some other more log
printed when commencing an AXFR.
Allow specifying a primary server and retrieve the zone's content.
Add tests and an Expired bool to zone struct, to stop server zones
that are expired. The zone is retrieved on Startup, no updates of
changed content are done. We also don't respond to notifies yet.
